Fe-Mg interdiffusion profiles in rimmed forsterite grains in the Allende matrix: Time-temperature constraints for the parent body metamorphism

The Allende matrix is dominated by micron-sized lath-shaped fayalitic olivine grains with a narrow compositional range (Fa(40-50)). Fayalitic olivines also occur as rims around forsterite grains in chondrules and isolated forsterite fragments in the matrix or as veins cross-cutting the grains. Allende is a type 3 CV carbonaceous chondrite having experienced a moderate thermal metamorphism. There is therefore a strong chemical disequilibrium between the large forsterite grains and the fayalite-rich fine-grained matrix. Chemical gradients at interfaces are poorly developed and thus not accessible using conventional techniques. Here, we used analytical transmission electron microscopy to study the microstructure of the fayalite-rich matrix grains and interfaces with forsterite fragments. We confirm that fayalitic grains in the matrix and fayalitic rims around forsterite fragments have the same properties, suggesting a common origin after the accretion of the parent body of Allende. Composition profiles at the rim/forsterite interfaces exhibit a plateau in the rim (typically Fa(45)), a compositional jump of 10 Fa% at the interface, and a concentration gradient in the forsterite grain. Whatever the studied forsterite grain or whatever the nature of the interface, the Fe-Mg profiles in forsterite grains have the same length of about 1.5 mu m. This strongly suggests that the composition profiles were formed by solid-state diffusion during the thermal metamorphism episode. Time-temperature couples associated with the diffusion process during thermal metamorphism are deduced from profile modeling. Considering the uncertainties on the diffusion coefficient value, we found that the peak temperature in Allende is ranging from 425 to 505 degrees C.